Question

What is the formula mass of $ 2 - butene $ , $ {C_4}{H_8} $ ?

Answer 1

Hint: In this question, first we have to find the atomic weight of atoms present in the molecule for which we have to know about the atomic masses of the atoms. After that we will substitute these values in the formula of formula mass.
Formula mass = $ {M_a} + {M_b} + {M_c} + ...... $
Where, $ {M_a} $ is the atomic mass of $ A $ atom
 $ {M_b} $ is the atomic mass of $ B $ atom
 $ {M_c} $ is the atomic mass of $ C $ atom.

Complete Step By Step Answer:
Formula mass of the compound is the sum of the atomic mass of the atom in the empirical formula of the compound. It is also known as the formula weight of the compound. Formula weight is given in amu, i.e. atomic mass units. For example, let us calculate the formula mass of glucose, i.e. $ {C_6}{H_{12}}{O_6} $ whose empirical formula can be written as $ C{H_2}O $ . So, the formula mass of glucose is:
 $ 12 + 2\left( 1 \right) + 16 = 30\;amu $
First of all we have to know about the atomic weight of each atom present in the compound. Thus, the atomic weight of the atoms is:
Atomic mass of carbon= $ 12 $
Atomic weight of carbon= $ 12 \times 4 \Rightarrow 48 $
Atomic mass of hydrogen= $ 1 $
Atomic weight of hydrogen= $ 1 \times 8 \Rightarrow 8 $
In this question, the molecular formula is equal to the empirical formula. Thus, the empirical formula is $ {C_4}{H_8} $ . As we know the formula is the sum of atomic weight of the atoms present in the molecule, so the formula mass is:
Formula mass of $ 2 - butene $ = $ 48 + 8 $
 $ \Rightarrow 56 $
Thus, the formula mass of $ 2 - butene $ is $ 56 $ .

Note:
Remember that to find the formula mass, we have to write the given compound in its empirical formula. After that we will find the atomic weight of each element present in the compound. Thus at last we will substitute the value in the formula.